Nature a challenge for bridge builders

Posted: Thursday, December 14, 2006

A period existed in the United States when riverboat captains who ruled the Mississippi River didn’t want any bridge abutments interfering with their waterway.

Westward expansion was at a standstill until a young lawyer named Abraham Lincoln, representing railroad interests, engaged the captains in court and prevailed, ultimately permitting bridges to be built across the “Mighty Mississippi.”

A similar stubbornness is being demonstrated over the spanning of the Kenai River in Soldotna today, but this time, the formidable force is that of nature, not riverboat captains.

When work on the Sterling Highway bridge project got under way in the spring of 2005, it was thought traffic would be moving across a shiny new bridge by autumn this year.

Motorists, however, are still driving by on a temporary bridge, catching sideways glimpses of work on the new bridge as they can.

It’s not for a lack of trying, according to the project manager for Wilder Construction, the company building the bridge.

Talking to about 20 community members during a Kenai Watershed Forum winter speakers series meeting at the Kenai River Center on Tuesday night, Scott Harter said once Wilder’s big crane was assembled on site and riverbank brush was grubbed out, workers “made it across the river in 18 days” with the temporary bridge.

A wooden deck was installed on the temporary bridge, fabric was stretched across the wood and the bridge was paved, making way for traffic to be diverted off the old bridge, he said.

Using several hundred projected photos, Harter gave the audience a behind the scenes look at work going on out of view of passersby.

Wilder crews needed to dig a 26-foot deep hole on one side of the river for a Soldotna city lift station, used to move sewage to the city’s treatment plant; they had to build a large sedimentation basin along Riverside Drive west of the Kenai River Lodge to treat storm runoff water, formerly discharged into the river; and they needed to build a trestle work platform out in the middle of the river to facilitate the crane used to drive steel piling sheets and remove steel girders from the old bridge.

“When the crane is holding the hammer, it’s close to 400,000 pounds,” said Harter in describing the scope of the work platform alone. The hammer he referred to is the large piece of equipment hoisted by the crane to drive steel pilings into the riverbed.

Two storm drain lift stations also were installed.

Once the work trestle was built, work began on the coffer dam, which was needed to provide a dry hole in the middle of the Kenai River for workers attempting to remove the old bridge center support and to build the new center pier.

That’s when trouble began.

No matter what method they tried, workers could not get steel sheets driven down into the extremely dense sedimentary material on the river bottom, Harter said.

“That was the main cause of the project delay,” he said.

Many of the sheets could only be driven three feet into the dense silt layer, he said, but eventually the footer for the new bridge was built 12 to 15 feet deeper than the old footer.

When the coffer dam was finally built, workers began pumping water out of the hole, but learned that despite using several pumps ranging in size from 18-inch to three-inch, the water kept coming in.

Undaunted, Wilder crews drove wedges between sheet piles to make them fit more tightly together and pumped grouting into the gaps to stop the leaks.

The river water never yielded totally, but enough leaks were stopped to reduce the amount of water coming into the coffer dam so it could be pumped out while workers worked.

Then a large ice jam that had formed down river from the bridge site broke up last winter, allowing upstream ice to flow.

“A big ice chunk hit the work trestle so hard, the crane operator got off and came ashore,” Harter said. “That’s when I ordered work to stop. It was a safety issue.”

In the spring, crews came back, faced with having to remove ice from inside the dam.

“The ice was four to six feet thick,” Harter said.

The work progressed; the footer was poured; so was the 33-foot tall center wall; and the hammerhead for the center pier.

Workers could then remove the steel piling sheets that had been driven into the riverbed, but because some were so bent and twisted from the intense hammering to get them in, they would never come back out.

“We sent a diver down to cut off the sheets (that could not be removed) below grade,” Harter said.

Then came the widely reported problem with the span girders.

Manufactured in Montana, barged to Seward and trucked to Soldotna, the six girder spans, when assembled, are each 416 feet long.

“After all the girders were set, we realized they had twists and the camber was too high,” Harter said.

“That was the second major delay,” he said.

Wilder decided to reengineer the project so the girders they had in place could be used.

When asked by an audience member Tuesday night what caused the twist, Harter said, “That question has not been answered.”

Work is continuing now on building the form for the actual concrete bridge deck.

“There will be 77 miles of steel (reinforcing) bar in the deck alone,” Harter said.

Because the coffer dam problems delayed the project six to seven months, Harter said concrete will be poured for the deck this winter.

“We had planned to pour in May,” he said.

“The biggest single challenge now is to get heat into the girders to heat the concrete,” Harter said.

Right now, Wilder has about 30 people working on the bridge 10 hours a day, seven days a week.

“We’ll be giving them Friday (Dec. 22) through Tuesday (Dec. 26) for a Christmas break,” he said. “They really deserve it.”

Phil Hermanek can be reached at phillip.hermanek

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